In paper manufacturing—a preferred application of roller bodies in accordance with the invention—rollers which are several metres in length and more than a metre in diameter are used to manufacture the finished paper web from cellulose sludge by means of thermal and mechanical treatment. Rollers made of a chilled casting, in particular clear chilled casting, or forged steel are used. The roller bodies made of a chilled casting are manufactured in a gravity die casting method, in most cases upright by static gravity die casting. The annular dies mean that a carbidic, white cast iron is achieved in the outer circumferential rim zone, the shell. The circumferential rim zone or shell solidifies metastably, white, and the carbon there is bound in the form of carbides. Stable solidification occurs in the core, where the molten mass solidifies grey and the carbon occurs as free graphite in the iron matrix. The required hardness at the outer circumference of the roller body, the surface hardness, is ensured by the material of the shell—the white cast iron. The hardness at the surface and in the near-surface depth range is set via the die and the alloy elements of the iron base molten mass. Negative effects of a clear chilled casting include the impact brittleness, a sensitivity to sudden changes in temperature, and uneven wear at the outer circumference of the roller due to the carbides contained in the white cast iron.
In order to overcome said disadvantages, EP 0 505 343 A1 proposes casting the roller body from an iron base alloy, such that a pearlitic or ferritic-pearlitic micro-structure is created which is at least 60% pearlitic. The iron base alloy contains 3.0% to 3.8% carbon, 1.5% to 3.0% silicon and 0.5% to 0.9% manganese. Maximum amounts for phosphorus and sulphur are specified. Chromium, nickel, copper, magnesium, molybdenum, tin or aluminium are used as additional alloy elements. The cast roller body is surface-hardened—induction and flame hardening are mentioned—and tempered after the martensitic transformation, such that the roller body obtains a tempered martensitic structure in its circumferential rim zone. The martensitic structure of the circumferential rim zone is associated with a considerable danger of fractures.
Using the alternative of roller bodies made of forged steel, as mentioned at the beginning, it is possible to solve said material problems. The surface hardness and hardness penetration depth of the roller body are set by subsequent thermal surface treatment. It is however manufactured from a forging grade ingot, the weight of which depends on the size of the roller body. Roller bodies such as the invention relates to weigh many tonnes—large roller bodies for example have a weight of about 50 t or even more. The weight of the forging grade ingot for such roller bodies can be up to 200 t. In this weight range, hollow-forging is only possible at very great cost. This additionally makes great demands on the interior quality of the forged steel with regard to flaws, inclusions and the like. The yield is therefore very low.
It is an object of the invention to provide, at a favourable price, a roller body having improved mechanical properties as compared to a clear chilled casting. The roller body shall be able to replace the known roller bodies made of a clear chilled casting and shall in particular exhibit the required hardness at its surface and also in the near-surface depth range, but not the unevenness of wear and impact brittleness which are disadvantageous in applications. The danger of fractures which is associated with a martensitic shell shall likewise be avoided.